1. Field of the Invention
The present invention relates to a document reading device suitable for the purpose of preventing forgery of paper money (bills), marketable securities, etc., and to an image forming apparatus having such a document reading device.
2. Description of the Related Art
With recent improvements in performance of color copiers, color printers, and the like, the possibility that paper money, marketable securities, gift certificates, admission tickets for entertainments, etc. are forged has increased. A technique for preventing such forgery is known in which image patterns of paper money etc. are previously stored in a memory, image data of a document is compared with the contents of the memory, and when they are close to each other, a normal image output operation is not performed (for example, the corresponding portion is printed black). Another technique is known which causes a printed image to contain a dot pattern that represents the production number or the like of a copier used but cannot be recognized by the naked eye, to thereby facilitate tracing of a forger.
However, it is difficult for the former technique to cope with all the documents of which forgery should be prevented. Namely, although the kinds of paper money circulated are limited, there are enormous kinds of marketable securities, admission tickets, and the like. It is not realistic to store image patterns of all of such items. Further, although the latter technique facilitates tracing of forgery after it is done, that technique cannot prevent the forgery itself.
On the other hand, a technique is known in which the genuineness of a bill, for instance, is judged based on metal fibers that are contained in it in a predetermined distribution pattern (Japanese Unexamined Patent Publication Nos. Sho. 63-501250, Hei. 6-87288, and Hei. 6-79991). According to this technique, a slit is formed so as to cross a waveguide which propagates microwaves. A bill, for instance, is inserted into the slit while the microwave transmittance is measured. If the bill is genuine, the transmittance should vary in accordance with the above-mentioned distribution pattern of metal fibers. Therefore, the genuineness of the bill is judged based on whether such a variation is obtained.
Although various kinds of paper money etc. have different distribution patterns of the metal fibers, they have a common feature that they reflect microwaves at at least a certain level of reflectance. Therefore, a document reading device used in a copier or the like may be so constructed that microwaves are applied to a document, and the document is read only when it does not reflect the microwaves. However, this configuration has various problems described below.
First, since the document reading device uses an exposure lamp or the like, it is likely subject to a temperature variation. Therefore, the output level of a microwave transmitter/receiver circuit largely varies through its temperature characteristics, the size of a microwave transmission line largely varies due to thermal expansion, and an output variation due to aging is also large. As a result, there may occur a case that the microwave transmitter/receiver circuit does not produce a normal output. This will cause various problems such as the forgery preventing function not being effected, or being effected for a general document. Further, a user may not repair the document reading device in spite of a failure of the forgery preventing function. Or he may intentionally makes the forgery preventing function inoperative and misuses the document reading device. For example, he may covers a microwave transmitter section with a metal plate.